The present invention relates to a self-heating type ignition plug (SHIP) which can be widely used, and more particularly to an ignition plug of a type in which multiplication of electric heating and self-heating actions is effected to improve thermal efficiency to reduce power consumption, in which fuel is evaporated and ignited as soon as possible to ensure stable and smooth combustion and in which construction is simplified to improve productivity and durability while reducing production cost.
According to prior art, in a compression ignition engine, such as a Diesel engine, fuel is fed under high pressure in an atomized state into a combustion chamber in an engine cylinder so that the atomized fuel may be brought into contact with highly-compressed hot air, thereby effecting its spontaneous combustion. However, when the temperature of ambient air is low, intake air cannot be heated to a sufficiently high level even after it has been compressed, thus making it difficult to ensure combustion and to start the engine. In order to facilitate ignition of fuel, therefore, the engine combustion chamber is equipped with a heating plug, which is heated with electric power, and an auxiliary ignition plug which is called a glow plug. The heating plug of this type is a kind of an electric heating plug and is divided into a type in which an exothermic element of electric resistance type is exposed directly to the outside and into a type in which the exothermic element is covered through an insulating substance with a protecting metal. The surface temperature is raised to 800 to 1000.degree. K. by a power supply. The heating plug of this conventional type is energized prior to starting the engine, thus preheating air in the combustion chamber at the end of the compression stroke and promoting ignition of atomized fuel by the hot inner wall of the combustion chamber due to the preceding combustion. However, a multi-cylinder engine is equipped with a corresponding number of conventional heating plugs, each requiring a current of 10 amperes during the heating operation. Therefore, continuous use of the heating plugs is limited by the capacity of the battery to a period of from 30 to 120 seconds. Immediately after the engine starts, moreover, the temperature of the inner wall of the combustion chamber is so low as to establish a remarkably long ignition delay from the injection and to the ignition of the fuel. More specifically, an engine which is equipped with a conventional heating plug and which has been used in experiments conducted by the Inventors produces such an abnormal combustion cycle [which is indicated at solid curve A in FIG. 1 plotting the temperature (.degree.K.) of the combustion gases against the crank angle] that ignition (IG) is experienced far after the top dead center (TDC) and immediately before the bottom dead center (BDC), while generating high noises. Incidentally, broken curve B in FIG. 1 indicates the normal combustion cycle. In the preceding abnormal combustion cycle the engine cannot generate its expected output but, still the worse, discharges white smoke due to unburned fuel as a result of incomplete combustion. The condition thus far described is continued for several or more minutes after the engine starts before the wall temperature of the combustion chamber is heated up. From the experiments conducted by the Inventors with the use of a conventional heating plug, moreover, it has also been revealed that the surface temperature of the heating plug after the engine starts has such a tendency as is indicated in solid line C in FIG. 2, in which the surface temperature of the heating plug is plotted on the ordinate against the running time (in minutes) of the idling from the engine start plotted on the abscissa so that the variation in the surface temperature of the heating plug may be illustrated. In FIG. 2, the period of power supplying time is indicated in a character PS. As shown, if the power supply to the heating plug is interrupted, the surface temperature thereof is abruptly dropped, but is gradually elevated, as the combustion in the combustion chamber reaches the normal condition, until it is stabilized about 14 minutes later. This time becomes more or less different in accordance with the running conditions of the engine, cooling water temperature or ambient temperature. It is also confirmed by the experiments of the Inventors that white smoke is discharged from the engine when the surface temperature of the heating plug is lower than 800.degree. K. In this respect, another experiment has been conducted by continuously supplying the heating plug with the electric power for about ten minutes or more, although this long a power supply is practically impossible due to the limited capacity of the battery. It has also been confirmed from the experiment that neither the white smoke nor the combustion cycle shown in FIG. 1 are sustained. In FIG. 2, the period of generating time of the white smoke is shown in a character WS.
Therefore, it has been desired that a heating plug of remarkably low power consumption be developed for practical use.
On the other hand, the conventional heating plug is so constructed that a protecting metal tube is heated through an insulating substance by a resistive exothermic element disposed therein. In order to hold the protecting metal tube at a necessary temperature, consequently, the exothermic substance itself has to be held at a considerably high temperature, requiring a substance having a high melting point. As a material satisfying the required condition, various kinds of substances have been developed, each has a low resistivity so that it has to be machined into a wire having a practical resistance and a preset exothermic capacity. A conventional heating plug has the following drawbacks: its construction is so complicated that its productivity is hampered and it is unduly susceptible to accidental breakage of the wire.